1. Field of the Invention
The present invention relates to a method for making an improved hermetic electrical inductive device for use in an hermetic atmosphere which incorporates halogenated methane compositions such as refrigerants, and more particularly to a method for making hermetic devices including coils wound of magnet wire having resistance to blistering when exposed to R-22 or like refrigerants under saturation conditions at temperatures up to and above about 210.degree. C.
2. Brief Description of the Prior Art
Stalled or stopped electrical inductive devices such as motors operating in hermetic atmospheres including refrigerants, for example R-22 or monochlorodifluoromethane, often reach temperatures as high as 200.degree. C. or 210.degree. C. during startups. Field failures generally exhibit blistered wire windings, which have been observed to result from the refrigerant in the film being exposed to the high temperatures.
A wide variety of polymer varnish or enamel coatings for magnet wire, including polyesters, polyesterimides, and polyimides, are well known and widely used. See, for example, U.S. Pat. No. 3,652,500, issued Mar. 28, 1972, to M. A. Peterson for "Process for Producing Polyamide Coating Materials by Endcapping"; U.S. Pat. No. 3,663,510, issued May 16, 1972, to M. A. Peterson for "Process for Producing Polyamide Coating Materials"; U.S. Pat. No. 4,004,062, issued Jan. 18, 1977, to M. A. Peterson for "Aqueous Polyester Coating Composition;" and U.S. Pat. No. 4,073,788, issued Feb. 14, 1978, to M. A. Peterson, for "Partially Imidized Polyamide-acid Polymers, Aqueous Coating Compositions, Coated Wire and Method, and Partially Imidized Intermediate." The disclosures of the foregoing patents are incorporated herein by reference and made a part hereof. In general the polyesterimide, polyester, and polyamideimide overcoated polyesters, described in the aforementioned and other patents do not exhibit blister resistance when exposed to halogenated methane and ethane refrigerants, particularly the methane or R-22 refrigerants, in an hermetic environment at temperatures in the range of about 150.degree. C., to about 210.degree. C. While many of the polyimides described in the aforementioned patents do exhibit blister resistance when saturated with the refrigerant composition at high temperatures, they are not cost effective, both from the standpoint of the raw materials utilized and the solvent utilized.